LECT. 

 X. 



328 FERGUSON'S LECTURES. 



, PROBLEM VIII. 



An easy method for folding the hour of the night by any 

 two known stars, without knowing either their altitude 

 or azimuth ; and then, of finding both their altitude and 

 azimuth, and thereby the true meridian. 



Tie one end of a thread to a common musket bullet ; 

 and,, having rectified tlie globe as above, hold the other 

 end of the thread in your hand, and carry it slowly 

 round betwixt your eye and the starry heaven, until you 

 find it cuts any two known stars at once. Then, guess- 

 ing at the hour of the night, turn the globe until the in- 

 dex points to that time in the hour circle : which done, 

 lay the graduated edge of the quadrant over any one of 

 those two stars on the globe, which the thread cuts in the 

 heavens. If the said edge of the quadrant cuts the other 

 star also, you have guessed the time exactly ; but if it 

 does not, turn the globe slowly backwards or forwards, 

 until the quadrant (kept upon either star) cuts them 

 both through their centers : and then, the index will 

 point out the exact time of the night ; the degree of the 

 horizon, cut by the quadrant, will be the true azimuth 

 of both these stars from the south ; and the stars them- 

 selves will cut their true altitudes in the quadrant. At 

 which moment, if a common azimuth compass be so set 

 upon a floor or level pavement, that these stars in the 

 heaven may have the same bearing upon it (all owing for 

 the variation of the needle) as the quadrant of altitude 

 has in the wooden horizon of the globe, a thread ex- 

 tended over the north and south points of that compass 

 will be directly in the plane of the meridian : and if a 

 line be drawn upon the floor or pavement, along the 

 course of the thread, and an upright wire be placed in 

 the southmost and of the line, the shadow of the wire 

 will fall upon that line, when the sun is on the meridian, 

 and shines upon the pavement. 



